1. Field of the Invention
The subject matter of the present invention is ionic perfluorovinyl compounds, the polymers obtained from these compounds and their applications.
2. Description of the Prior Art
Polyelectrolytes of polyanion type incorporating functional groups of sulfonate or carboxylate type are known as ion-exchange resins (polyacrylic acid, polystyrenesulfonic acid optionally crosslinked with divinylbenzene). These polyelectrolytes are dissociated solely in the presence of water or of highly polar protic solvents, such as polyalcohols, for example ethylene glycol or glycerol. The corresponding acid groups (carboxylic or sulfonic acids) do not exhibit marked catalytic properties due to the absence of swelling of the resin and the strong association of ion pairs. In the form of membranes, these polymers only have a mediocre stability under the operating conditions of a hydrogen-air fuel cell; in particular, they are rapidly degraded by oxidizing species present on the oxygen electrode side. Likewise, these polymers cannot be used in membrane processes, such as the chlorine-sodium hydroxide electrochemical process.
Furthermore, perfluorinated membranes (Nafion®) carrying sulfonic groups are known which exhibit good chemical stability under the operating conditions of a fuel cell and for the chlorine-sodium hydroxide process. These materials are copolymers of tetrafluoroethylene (TFE) and of a comonomer carrying sulfonyl functional groups. However, the impossibility of crosslinking these polymers requires that the density of ionic groups be kept low, in order to prevent the resulting polymers from being excessively soluble or swollen by water, resulting in a mediocre mechanical strength and a relatively limited conductivity. Furthermore, these membranes exhibit a high permeability to gases (oxygen and hydrogen) and to certain solvents, such as methanol, which is harmful to the energy efficiency of fuel cells, more especially those of methanol-air type (“crossover”). Furthermore, although the sulfonate groups attached to perfluorinated groups are partially dissociated in aprotic solvents and although the solvating polymers are particularly advantageous for secondary batteries in which the reactions at the electrodes involve lithium ions, the conductivity of the gels obtained by swelling Nafion® membranes with aprotic solvents, alone or as a mixture, and the conductivity of the mixtures of these polyelectrolytes with polyethers based on ethylene oxide remain too low. Furthermore, the significant fraction of perfluorinated segments —CF2CF2— resulting from the TFE comonomer makes these compounds sensitive to reduction at potentials close to those of the negative electrode, resulting in the polymer being destroyed. Moreover, the chemistry of these polymers is complex and expensive, and the yield in the manufacture of the monomer of perfluorovinyl ether type:CF2═CF—O—[CF2CF(CF3)]p—O—CF2CF2SO2F, 0≦p≦5by thermal cracking of perfluoropolyethers-acid fluoride obtained by addition of CF3CF═CF2 to isomerized sultones is low and limits the use of these materials.
Polymers which comprise anions attached to the backbone of the polymer and which are optionally plasticized or gelled by a solvent of polar type are of great advantage in electrochemical systems, such as primary or secondary batteries, supercapacitors or systems for modulating light (electrochromic windows). Such polymers are mainly derivatives of ethylene oxide, of acrylonitrile, of polyesters of alkyl or oxaalkyl acrylate or methacrylate type, or of vinylidene fluoride. The production of monomers carrying highly delocalized anionic functional groups which can be incorporated, either by copolymerization or by cocrosslinking or alternatively by mixing polymers, in macromolecular materials such as those used in the electrochemical systems described above is therefore highly advantageous. The ionic monomers described above as components of membranes of Nafion® type cannot be suitable for this use because the high fraction of perfluorinated segments necessary in order to obtain the maximum conductivity in aprotic media (≈1M.1−1) corresponds to a decreased dielectric constant in the vicinity of the ions and to an increased segmental stiffness, which are unfavorable to the movement of the ions. Moreover, the sulfonate groups are insufficiently dissociated in comparison with the salts of anions delocalized from nitrogenous centers or from carbon, such as, for example, the anion corresponding to the formula (RFSO2)X(SO2R′F)—, in which X is N, C—R or C—SO2R″F, RF, R′F and R″F are chosen from fluorine and fluorinated monovalent groups, or else RF and R′F form the components of a divalent ring, and R═H or any monovalent organic radical.
W. Navarrini et al. (U.S. Pat. No. 5,103,049) disclose methods for the preparation of Rf—CF═CF—SO2F compounds in which Rf is F or a perfluoroalkyl group comprising 1 to 9 carbon atoms. Among these compounds, only CF2═CF—SO2F is capable of acting as basis for monomers which can polymerize without steric constraints. However, this material has been shown to be too reactive to act as a precursor for monomer salts and anions, because the nucleophilic addition to the C═C double bond, which is depleted in electrons both by the fluorine atoms and by the —SO2F group, generally takes place more rapidly than the substitution of the fluorine of the SO2F group, thus preventing access to ionic monomers or to precursors of ionic compounds (“Studies of the Chemistry of Perfluorovinylsulfonyl Fluoride”, Forohar Farhad, Clemson University, Thesis 1990 UMI 9115049). In particular, the methods for the preparation of anionic compounds used with RFSO2F compounds cannot be applied to the compound CF2═CF—SO2F. A process which consists in attaching a perfluorovinyl group to a phenyl nucleus carrying an SO2F group has been provided by C. Stone et al. (WO/96/39379); however, in this case too, it is not possible to convert the CF2═CF—C6H4SO2F molecule to an anionic monomer because of the sensitivity of the CF2═CF— group, which is more reactive than —SO2F with respect to bases of OH− or NH3 type.
Another process for the preparation of monomers of the TFE type comprising an anion has been provided by D. Desmarteau et al. (U.S. Pat. No. 5,463,005). It consists in preparing a compound comprising a perfluorovinyl group and an SO2F group, in protecting the perfluorovinyl group, for example by addition of Cl2, in converting the SO2F group to an ionic group and in then deprotecting the perfluorovinyl group. Such a process is nevertheless lengthy and expensive and the polymers obtained from said monomers exhibit the same disadvantages as the polymers of Nafion® type, resulting from the low content of SO3− or sulfonimide ions in the absence of crosslinking.
The aim of the present invention is to provide a novel family of ionic compounds, which compounds exhibit extensive delocalization of the negative charge and good activity in polymerization or in copolymerization and allow the preparation of macromolecules possessing dissociated and stable ionic functional groups, and a process for the preparation of these compounds from fluorinated derivatives which are commercially available at low cost, for example hydrofluorocarbons or halocarbons.
For this reason, the subject matter of the present invention is ionic monomer compounds, the homopolymers and the copolymers obtained from these compounds, their applications and a process for their preparation.